Basic Calculator II

题目：

 

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

基本思路：     把减法当成负数入栈，把正数的数值全部入栈，遇到*/运算先计算后将结果入栈，最后栈中的所有数值依次取出加法运算

public class Solution {
    public int calculate(String s) {
        
        if (s != null && s.length() != 0) {
            Stack<String> stack = new Stack<>();
            int result = 0;
            int i = 0;
            while (i < s.length()) {
                if(s.charAt(i) == ' ' || s.charAt(i) == '+'){
                    i++;
                }
                else {
                    char tempChar = s.charAt(i);
                    StringBuffer buffer = new StringBuffer();
                        if(tempChar == '*' || tempChar == '/'){
                        int tempResult;
                        String firstVal = stack.pop();
                        StringBuffer secondVal = new StringBuffer();
                        int j = i+1;
                        while(s.charAt(j) == ' '){
                            
                            j++;
                        }
                        secondVal.append(s.charAt(j));
                        while(j+1 != s.length() && s.charAt(j+1) > 47 && s.charAt(j+1) < 58){
                            secondVal.append(s.charAt(++j));
                        }
                        if(tempChar == '*')
                            tempResult = Integer.parseInt(firstVal) * Integer.parseInt(secondVal.toString()) ;
                        else {
                            tempResult = Integer.parseInt(firstVal) / Integer.parseInt(secondVal.toString()) ;
                        }
                        stack.push(Integer.toString(tempResult));
                        if(j>i){
                            i=j;
                            i++;
                        }
                            
                        else {
                            i++;
                        }
                    }
                    
                    else{
                        buffer.append(s.charAt(i));
                        if(i < (s.length()-1)){
                            int j = i+1;
                            while(j < s.length() && s.charAt(j) == ' '){
                                
                                j++;
                            }
                            while(j != s.length() && s.charAt(j) > 47 && s.charAt(j) < 58){                    
                                buffer.append(s.charAt(j++));
                            }
                            if(j>i){
                                i=j;
                            }
                                
                            else {
                                i++;
                            }
                        }else{
                            i++;
                        }
                        stack.push(buffer.toString());
                    }
                }
            }
            while(!stack.isEmpty() && stack.size() != 1){
                String secondVal = stack.pop();
                String firstVal = stack.pop();
                int tempResult = 0;
                tempResult = Integer.parseInt(firstVal) + Integer.parseInt(secondVal.toString());
                stack.push(Integer.toString(tempResult));
            }
            return Integer.parseInt(stack.pop());
        }
        return 0;
        
    
    }
}